Prognostic factors were studied in a series of 80 patients with aplastic anaemia. The most important and easily obtainable individual variables, isolated in a previous univariate statistical analysis, were placed in a linear logistic regression model. A prognostic formula was derived containing the following 4 variables, in decreasing order of significance: (1) reticulocytes, (2) interval from onset of symptoms to first visit, (3) mean red cell volume, and (4) platelets. The prognostic formula permits an estimate of the probability of death within 3 months from the first visit. Its usefulness in several clinical situations, especially those related to bone marrow transplantation, is illustrated.